1. Field of the Invention
This invention relates to the preparation of a sustained release fertilizer composition. More especially, this invention relates to the treatment of a water-soluble granular fertilizer composition to coat the same with a coating material whereby the final composition will have sustained release fertilizing properties. This invention is particularly directed to a method of pre-treating granular fertilizer compositions whereby the granular composition has the voids in its exterior surface filled with a generally nutrient fertilizer composition. The resultant inter-product comprises enveloped granules of the water-soluble granular material and is later on coated with a known covering material such as wax or a plastic material or preferably molten sulfur.
2. Discussion of the Prior Art
The coating of granular water-soluble fertilizer compositions is broadly known. Generally, these compositions are coated with a water insoluble or sparingly soluble inert substance to effectively seal the surface of the composition. The granulated fertilizers which are employed are, however, ones which have a relatively large specific surface area inasmuch as the "sweating" which occurs during their formation results in the formation of fissures and holes in the surface of the particles. Hence, the specific surface area of the granules is substantially larger than it would be if the particles themselves had a spherical shape. In the coating of such conventional fertilizer granules more coating material must be employed. Otherwise, the composition will be more readily attacked by water and will have a shorter period of fertilizer release.
It therefore, became desirable to provide a process for enveloping fertilizer granules whereby the fertilizer granules could be coated by a process with almost the same consumption of coating material for producing grains within a narrow range of size distribution by the method thus developed, simultaneously it was able to lower the amount of enveloping material, necessary for the desired retarding effect of releasing the various plant nutrients.